The requirement for isolation of both radio frequency (RF) and thermal energy has existed since the beginning of the electronic age. For every circuit that generates these two forms of energy, there is a desire to isolate this energy from the external environment in which that circuit operates. In general, it is highly desirable to minimize the effect that a given circuit has on its environment as well as the effect that the environment has on the circuit. This is especially true in communication system applications where high levels of RF radiation and thermal energy, in the form of heat, are commonplace.
Typically, there are three areas which are of primary concern to the RF circuit designer. They are to provide adequate RF isolation (on the order of 25-35 dB attenuation), adequate dissipation of heat, and sufficient contact pressure between the enclosure system and the circuit ground plane. These three characteristics require the system to be designed with exacting precision and flexibility to accommodate changes in the systems environment. Keeping in mind that the environment in which the enclosure system resides is itself a dynamic system, it is of paramount importance that consideration be given specifically to a few key parameters. Mechanical tolerance build-up, or mechanical stack-up, is a common problem which warrants thoughtful design consideration. Mating piece parts of known dimensions in a predetermined way, even in a highly manufacturable process, yields mechanical tolerance build-up, which has an undesirable, perhaps disastrous, impact on the performance of the final product. Examples of this may include piece parts which are worst case in terms of mechanical tolerance, even slightly convex or concave, or any combination of the two. Of course, the more piece parts that a given enclosure system design requires, the more subject it is to the consequences of this undesirable phenomenon. Clearly, the issue of mechanical tolerance build-up is among the top of all assembly process problems, and also a costly one when one considers the labor required to re-work or scrap the units which don't meet final specifications. Also, the stresses to which the enclosure system is subject during operation may serve to change the physical characteristics which affect these critical system parameters. As is the case with most electronic circuits, the enclosure system for such circuits is subject to thermal stresses, vibration, shock pulses, and often, RF energy emitted from a circuit external to the enclosure system. All of the aforementioned conditions need to be addressed in the design of an enclosure system for optimal performance regarding isolation of the target circuit.
Typically, shielding of this type is provided by a conductive metal enclosure which is held at a fixed electrical potential. Ordinarily, the enclosure has two portions; the first portion defines the housing for the electronic circuitry, while the second portion of the enclosure acts as a cover shield. Since the required spacing between points of electrical contact is proportional to the wavelength of the signal being attenuated, the spacing requirements depend on the frequency of the undesired RF radiation. Specifically, spacing between points of electrical contact along the joint of the cover and housing need only be less than the 1/20th wavelength of the RF radiation frequency. At high radio frequencies (i.e., MHz and GHz) the associated 1/20th wavelength of an electromagnetic wave becomes sufficiently short so that even a small gap in the electrical and physical connection of two separate pieces of housing can be enough to allow a leak of the high frequency electromagnetic radiation through the shield. At lower RF frequencies this is not a substantial problem, since the 1/20th wavelengths of the lower RF frequencies are of a physical length such that spacing between good electrical contact points where the two housing parts mate, are rarely sufficiently large enough to let the relatively long 1/20th wavelength of low frequency RF leak through the shield.
U.S. Pat. No. 4,831,498, "Shield Structure for Circuit on Circuit Board", shows the use of a continuous rib structure on a cast cover member which makes physical contact with a conductive pattern on the circuit board. This type of structure is either difficult to precisely manufacture, for example within flatness specifications, due to casting tolerances, or very costly if one should attempt to machine this part. Furthermore, a non-compliant rib structure, such as the one employed, is likely to lose contact with the conductive pattern over time. This may be due to warping or any other effect which alters the flatness characteristic of the either the cover, the circuit board, or both. Additionally, the issue of heat dissipation is not addressed by such an enclosure.
U.S. Pat. No. 4,384,165, "Radio Frequency Shield with Force Multiplier Interconnection Fingers for an Electromagnetic Gasket", shows the use of highly resilient finger projections on the cover piece being forced against the mating enclosure piece to make electrical contact between the two pieces. While this solution overcomes the problem of maintaining contact pressure through the use of resilient fingers, these fingers are both costly, and difficult to manufacture, perhaps even impossible for applications approaching 1 GHz and beyond. The issue of heat dissipation is also not addressed by this design.
Another attempted solution, a portion of which is shown in FIG. 2, uses a conductive wire mesh, or RF braid, to form an electromagnetic shield around the target circuit. The enclosure consists of a housing 23 and a flat planar cover plate 25. The housing 23 includes flat planar shoulder 23A along the perimeter of an access opening 26 into the housing 23. The housing shoulder 23A has a channel 29 cut into its surface. RF braid 27, a conductive wire mesh, is inserted into the channel 29 formed in housing shoulder 23A. The cross-sectional area of RF braid 27 is sufficiently great so that when it is fitted into channel 29 a significant portion of it is above the plane defined by housing shoulder 23A. Cover plate 25 is aligned over housing shoulder 23A by mating screw holes 31A in cover plate 25 and mating screw holes 31B in housing shoulder 23A. Screws 33 secure cover plate 25 flush against housing shoulder 23A. The RF braid 27 is compressed by the flush engagement of cover plate 25 with housing shoulder 23A. While this system has been shown to provide as much as 60 dB attenuation, the RF braid is a very costly piece-part and it also provides a significant challenge to the operator placing the RF braid in the channel during the assembly process. Furthermore, the electrical contact made between the cover shield and the RF braid is difficult to maintain over time due to diminishing contact pressure provided by the less-than resilient braid material. Therefore, in terms of a practical solution to providing approximately 30 dB isolation, the RF braid design falls considerably short of the mark.
Accordingly, there exists a dire need for a cost effective, easily manufacturable, effective means for isolating both RF and thermal energy emitted from an electronic circuit from an external environment in which the circuit resides. The enclosure system must also be designed with enough flexibility so as to have continued success through the operational life cycle of the electronic circuitry, whose environment is itself a dynamic system undergoing many changes.